net: ipa: stop using sizeof(status)

The IPA packet status structure changes in IPA v5.0 in ways that are
difficult to represent cleanly.  As a small step toward redefining
it as a parsed block of data, use a constant to define its size,
rather than the size of the IPA status structure type.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>